It's easy to go under the map and build, and kill ennemies, or return to the surface, or walk under the map, godmode, etc
The instructions are in the video in a screen at the start of the video in english and french

You get stuck under there. So you would essentially die regardless. Epic is also aware of the issue and there is a megathread for this on R/FortNiteBR. Players are also being reported for doing it and you'll likely be reported as well.
Patched.